How can I have a split column inside a box?
you can stack layouts, as each of the container layout elements can host other containers:

    layout = self.layout
    box = layout.box()

    # put a column layout into the box
    col = box.column(align = True)

    # also separators work
    col.separator()

    # add a row layout into the column, which is inside the box, which is inside a panel,.....
    row = col.row(align = True)
    row.operator(**example**)
    row.operator(**example**)